1. Field of the Invention
The present invention relates to a display device, and more particularly, to a dual panel type organic electroluminescent (EL) display device and a method of fabricating the same.
2. Discussion of the Related Art
An organic electroluminescent (EL) display device, which is a type of flat panel display, is a self-emission type display. In general, the organic EL display device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Accordingly, the organic EL display device does not require an additional light source and has a light weight, thin profile, and compact size.
The organic EL display device also has other excellent characteristics such as low power consumption, superior brightness, fast response time and simple fabrication process. As a result, the organic EL display device is regarded as a promising display for next-generation consumer electronic applications, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers.
There are two types of organic EL display devices: passive matrix type and active matrix type. While both the passive matrix organic EL display device and the active matrix organic EL display device have simple structures and are formed by a simple fabricating process, the passive matrix organic EL display device requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic EL display device is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic EL display device decreases. In contrast, active matrix organic EL display devices are highly efficient and can produce a high-quality image for a large display with a relatively low power.
FIG. 1 is a schematic cross-sectional view of an organic EL display device according to the related art. In FIG. 1, an organic EL display device 10 includes first and second substrates 12 and 28 attached to each other by a sealant 26 with a space therebetween. An array element layer 14 is formed on the first substrate 12 and includes a thin film transistor (TFT) T. In addition, a first electrode 16, an organic electroluminescent (EL) layer 18 and a second electrode 20 are formed on the array element layer 14. The first electrodes 16 is connected to the TFT T. The organic EL layer 18 may separately display red, green, and blue colors in each pixel region P.
The organic EL display device 10 is encapsulated by attaching the first substrate 12 to the second substrate 28. The second substrate 28 includes an absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the absorbent material 22 and the filled absorbent material 22 is fixed by a holding element 25.
FIG. 2 is a schematic circuit diagram of an array layer of an organic EL display device according to the related art. In FIG. 2, a gate line 36 is formed along a first direction, and a data line 49 is formed along a second direction intersected with the gate line 36, thereby defining a pixel region 30. A power line 62 also is formed along the second direction and spaced apart from the gate line 36. A switching element TS in the pixel region, and a storage capacitor CST is connected between the switching element TS and the power supply line 62. A driving element TD electrically connects the switching element TS to an organic EL diode DEL.
In particular, the storage capacitor CST is between a driving gate electrode 34 and a driving source electrode 52 of the driving element TD, as the driving element TD is a positive type transistor. The organic EL diode DEL is connected to the power line 62, and the driving drain electrode of the driving element TD may be connected to an anode of the organic EL diode DEL. The switching element TS and the driving element TD can be a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT.
When a scan signal is applied to a switching gate electrode 32 of the switching element TS from the gate line 36, an image signal is applied to the driving gate electrode 34 of the driving element TD through the switching element TS from the data line 49. The current density of the driving element TD is modulated by the image signal applied to the driving gate electrode 34. As a result, the organic EL diode DEL can display images with gray scale levels. Moreover, because the image signal stored in the storage capacitor CST is applied to the driving gate electrode 34, the current density flowing into the organic EL diode DEL is uniformly maintained until the next image signal is applied, even when the switching element TS is turned off.
However, when an array layer of TFTs and organic EL diodes are all formed on a single substrate, the production yield of an organic EL display device is determined by a product of the TFT's yield and the organic EL layer's yield. Since the organic EL layer's yield is relatively low, the production yield of the organic EL display device is limited by the organic EL layer's yield. For example, even when a TFT is properly fabricated, an organic EL display device can be determined to be unacceptable due to defects of the organic EL layer using a thin film of about 1000 Å thickness. Accordingly, this limitation causes loss of materials and an increase in production costs.
Organic EL display devices are classified into one of bottom emission-type organic EL display devices and top emission-type organic EL display devices based on a direction of light emitted from organic EL diodes. The bottom emission-type organic EL display devices are advantageous for their high image stability and variable fabrication processing due to encapsulation. However, the bottom emission-type organic EL display devices are not adequate for implementation in display devices that require high resolution due to the limitations of the increased aperture ratio.
On the other hand, since top emission-type organic EL display devices emit light along a direction upward of the substrate, light can be emitted without influencing the array layer that is located under the organic EL layer. Accordingly, the overall design of the array layer including TFTs may be simplified. In addition, the aperture ratio can be increased, thereby increasing the operational life span of the organic EL display.
However, since a cathode is commonly formed over the organic EL layer in the top emission-type organic EL display devices, material selection and light transmittance are limited such that light transmission efficiency is lowered. For instance, if a thin film type passivation layer is formed to prevent a reduction of the light transmittance, the thin film type passivation layer may fail to prevent infiltration of exterior air into the device.